Branches, Bifurcations and Retroflections over the Tail of the Grand Banks

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It is proposed to analyze existing data (hydrography, current meter, surface drifter, subsurface float, altimetric and SST observations), to characterize the mean and variable (mesoscale through seasonal to interannual) regional circulation around the Tail of the Grand Banks, ultimately enabling a more complete description of the branches, bifurcations and retroflections of the currents.This is a critical location for the North Atlantic circulation. There, major branches of the wind-driven (Gulf Stream and Labrador Current) and thermohaline circulations (Deep Western Boundary Current) meet, and dramatic changes in the character and pathways of the currents follow. The Labrador Current and Deep Western Boundary Current partially retroflect near the Tail of the Banks, but the amount of transport lost by the combined currents and the details of the retroflection pathway are unknown. Similarly, south of the Grand Banks, the Gulf Stream approaches the Tail from the west with full-depth flow carrying large transport. At the Tail, the Gulf Stream splits into four branches, with a critical branch turning north across the Tail to feed the North Atlantic Current east of the Grand Banks. The exact partitioning of transport into separate branches remains unknown. The interaction between the deep-reaching currents appears forced and constrained by the shoaling topography of the Tail, resulting in a regional distortion of the thermocline into a cyclonic meander over the Tail a trough - remarkably stable in both space and time. Based on existing observations we hypothesize that the trough configuration represents a bottleneck for the approaching currents, limiting the strengths of the current branches crossing the Tail. This will be a quantitative description within the limits of existing data, and may point to critical new measurements that would tighten the constraints on the characterization. The output from high-resolution numerical models will also be examined with the goal of gaining insight into the dynamics of the Tail-induced bottleneck.